Climate change is one of the most serious challenges societies are currently facing. The amount of greenhouse gases emitted to the atmosphere has increased substantially, and the amount will continue to increase in the foreseeable future. One of the major greenhouse gases is carbon dioxide (CO2) due to the use of fossil fuels (oil, natural gas, and coal), solid waste, trees and wood products, and also as a result of chemical manufacturing. The high demand for fossil fuel, which meets more than 98% of the world's energy needs, is largely responsible for the increase in the CO2 concentration levels in the atmosphere. The atmospheric CO2 concentration has risen to ˜280-390 ppm, which is an approximately 35% increase compared to atmospheric CO2 levels at the beginning of the industrial revolution. It is projected that atmospheric CO2 concentration will continue to increase unless effective CO2 emission control measures are taken.
Capturing CO2 emitted from power station flue gas has been considered to be a potentially effective approach to control atmospheric CO2 levels. Researchers have studied different methods for capturing CO2 in flue gas, such as cryogenic fractionation, membrane separation, and chemisorptions.
Industries are increasingly interested in the use of chemisorptions in particular for the separation of CO2 from flue gas because chemisorptions have been widely considered to be able to reduce energy consumption needed for separation of CO2 from flue gas. Chemisorptions may be performed via absorption or adsorption.
Adsorbents used for adsorption-based chemisorption processes include K2CO3 or Na2CO3, each supported on a nanoporous TiO(OH)2 support. K2CO3 supported on nanoporous TiO(OH)2 has been shown to increase CO2 sorption capacity per unit of K2CO3 compared to K2CO3 alone. Nanoporous TiO2 has also been shown to be a potent CO2 adsorbent. Macroporous TiO2 physically impregnated with monoethanolamine has also been shown as a potent adsorbent. Adsorption under the aforementioned conditions, however, may require increase of pressure in a range of 5-35 bar and decrease in temperature of the system within a range of 25-125° C. Furthermore, adsorbed CO2 must then be released from the sorbent, a process known as desorption, so that the sorbents may be reused. Desorption processes with sorbents K2CO3 and Na2CO3, each supported on a nanoporous TiO(OH)2 support, require high temperatures and/or high pressures of a gas introduced into the desorption system. Na2CO3 on nanoporous FeOOH has also been shown to be a potential CO2 adsorption/desorption system. However, CO2 adsorption/desorption capacity and kinetics associated with use of Na2CO3 is a concern for any industrial scale-up of carbonate-related CO2 adsorption/desorption processes. Furthermore, because alkali metal carbonate adsorbents involve a proton transfer mechanism, the activation energy required for the corresponding desorption/regeneration processes using alkali metal carbonates is indeed a bottleneck for industrial scale-up of these processes.
Other potassium-based adsorbents, such as K2CO3/activated carbon (AC), K2CO3/TiO2, K2CO3/Al2O3, K2CO3/MgO, and K2CO3/zeolite, have also been used for CO2 adsorption. These potassium-based adsorbents are regenerable to some extent and have shown high CO2 capture capacity. However, K2CO3/Al2O3 and K2CO3/MgO, in particular, have shown poor regeneration abilities. In other words, the CO2 adsorption capacities of K2CO3/Al2O3 and K2CO3/MgO decrease considerably after a few cycles of CO2 adsorption/desorption, at temperatures lower than 200° C. With some potassium-based sorbents, temperatures as high as 350-400° C. may be necessary for complete CO2 desorption, however, doing so may decompose the chemical structure of the original sorbents.
Alternatively, absorption generally uses aqueous alkanolamine compounds [e.g., monoethanolamine (MEA)] as CO2 sorbents. MEA-based CO2 absorption allows capture of CO2 in, for example, natural gas. MEA on TiO2 solid support has been shown to be a potent absorbent with desorption temperatures of 90° C. Nonetheless, scale up of such an absorption process may be economically unfeasible because the energy consumptions associated with the absorption and desorption are too demanding. The energy consumptions are demanding because of the dilute CO2 characteristics of flue gas and because the amine solutions are aqueous. Aqueous solutions of amine absorbents are typically required for CO2 separation because of the corrosiveness of the amines. However, the presence of water of the solutions requires more energy input during, for example, desorption due to the high specific-heat-capacity and latent heat of vaporization of water. Indeed, typical amine solutions used by the natural gas industry for absorption processes may contain about 70 wt % water. Furthermore, demanding energy input during the CO2 desorption is also due to the very slow kinetics of CO2 removal from the sorbent(s). However, amine sorbents may not be thermally stable. Furthermore, amine sorbents may vaporize at the required desorption temperatures, unlike the alkali carbonate adsorbents described above. The aforementioned bottlenecks of desorption processes renders current absorption/desorption technologies uneconomical for industrial scale-up. Thus, there is a need in the art for enhanced CO2 desorption technologies and combination sorption/desorption technologies.